1. Field of the Invention
The present invention relates to ultrasonic transducers in general and in particular to a novel ultrasonic transducer assembly and a method of manufacturing and assembling the same in a manner calculated to reduce labor and increase uniformity and reliability.
2. Description of Prior Art
An ultrasonic transducer assembly typically consists of one or more thin slabs of piezoelectric ceramic material mounted behind a faceplate. A backing material may or may not be attached to the rear of the transducer and one or more layers of matching material may or may not be attached to the front of the transducer. The front and rear faces of the ceramic are covered with a conductive electrode. Heretofore, electrical connections have been made to these electrodes by means of solder or a conductive epoxy.
In operation, the faceplate provides an acoustic impedance transformation from the ceramic to the working medium, as well as protecting the ceramic and providing electrical isolation.
Typical simple designs use a faceplate thickness of approximately one fourth of an acoustic wavelength at the normal frequency of operation. The faceplate is made of a material whose acoustic impedance lies betweeen that of the ceramic and the working medium. This improves the impedance matching of the ceramic to the medium and increases the bandwidth of the transducer. This has the effect of increasing the efficiency of the transducer and sharpening the time response, which is important if the unit is used in the pulse mode. For continuous wave doppler ultrasonic devices, the signal bandwidth is nearly zero, so a backing material is not necessary.
At the frequencies used in medical doppler ultrasonic blood flow meters, e.g. 2 MHz, an acoustic matching faceplate one quarter of a wavelength thick is quite thin. A typical faceplate might be, for example, 5 to 20 thousandths of an inch thick. Not only has a faceplate this thin been difficult to fabricate using conventional techniques, but also the use of conventional techniques has provided little protection for the brittle ceramic transducer.
In the past, faceplates were made from plastic film, such as Mylar.RTM., but adhesion to the plastic and sealing of the edges of the plastic to the ceramic transducer has posed difficulties in practice. In alternative prior known techniques, faceplates have been made by machining a solid piece of plastic down to the required thickness or casting the layer onto the surface of the transducer and grinding it down to the desired thickness. While these methods provide a faceplate with hermetic sealing properties, they are difficult to accomplish and are expensive.
In still another prior known manufacturing technique, some manufacturers have simply cast the ceramic into a block of resin or epoxy of unknown thickness. This has resulted in acoustic characteristics and ultrasonic beam profiles which are uncontrolled and of poor quality.
Attachment of the electrical contact to the front face of the ceramic members has also given rise to difficulties, given the thin faceplate attached to the front side of the ceramic. One method is to solder a wire to each front face of the ceramic members, but the resulting "bump" on the front face causes the ceramic member to be inclined at an unpredictable angle relative to the faceplate. Silver epoxy has been used also to connect the front face of the ceramic members to a nearby conductor, but the method is very labor intensive. Some manufacturers have the ceramic fabricated with a silver electrode which warps over the edge and onto part of the rear face. This ceramic is much more expensive, and performance is poor.